(Reuters) - The U.S. military campaign against al Qaeda should not be seen as a conflict without end, the Pentagon's chief lawyer said on Friday in a speech that broached a rarely discussed subject among U.S. officials.

The Defense Department's highest-ranking lawyer, Jeh Johnson, predicted al Qaeda would some day be so "effectively destroyed" that the United States would no longer say it is in an armed conflict. A text of his remarks to be given at Oxford University in England was made available to Reuters in advance in Washington.

The U.S. government points to the existence of an armed conflict as the legal underpinning for the indefinite detention of the global militant group's members and allies and for drone strikes in places such as Pakistan.

Johnson's remarks could ignite a global political debate with arguments from both the left and the right.

The speech to the Oxford Union did not forecast when such a moment would arrive because al Qaeda and its affiliates in Yemen and elsewhere remain a danger, he said.

But Johnson tried to frame the discussion with what he called conventional legal principles rather than a new legal structure emerging from the September 11 attacks.

"Now that efforts by the U.S. military against al Qaeda are in their 12th year, we must also ask ourselves: How will this conflict end?" said Johnson, an appointee of U.S. President Barack Obama.

Johnson delivered the remarks in Oxford as prepared, a spokesman said.

Johnson has been mentioned as a possible U.S. attorney general to succeed Eric Holder in Obama's second term. A former New York corporate lawyer and federal prosecutor, he has been at the center of internal administration debates on national security since he arrived at the Pentagon as general counsel in 2009.

An "open end" to the conflict has been a defining feature of what then-President George W. Bush called a war on terror beginning with the September 11 attacks that destroyed New York's World Trade Center, damaged the Pentagon and killed 3,000 people.

Three days after the attacks, Congress authorized force against all "nations, organizations or persons" who planned them or who aided the planners.

'TIPPING POINT'

In courtrooms, congressional hearings and executive orders, U.S. officials with rare exceptions speak of a conflict with no obvious end point.

"I think one day they will be defeated, but it's not going to happen any time soon," Senator Lindsey Graham, a Republican, told The Huffington Post website this week.

U.S. Defense Secretary Leon Panetta, Johnson's boss, said in a November 20 speech that U.S. forces had decimated al Qaeda's core and made progress in Yemen and Somalia, but needed to avert militant gains in Mali and Nigeria.

On Thursday, Panetta said al Qaeda fighters are still trying to make inroads in Afghanistan, which was the group's primary base of operations in 2001.

By asking how the conflict would end, Johnson could provoke a public conversation that gets more specific.

"There will come a tipping point," he said in the speech, "a tipping point at which so many of the leaders and operatives of al Qaeda and its affiliates have been killed or captured, and the group is no longer able to attempt or launch a strategic attack against the United States," that al Qaeda will be "effectively destroyed."

"At that point, we must be able to say to ourselves that our efforts should no longer be considered an 'armed conflict' against al Qaeda and its associated forces."

Under that scenario, law enforcement and intelligence agents would go on pursuing individual militants or groups - even those who are inspired by al Qaeda's ideology - with the military in a reserve role.

"'War' must be regarded as a finite, extraordinary and unnatural state of affairs," Johnson said. "We must not accept the current conflict, and all that it entails, as the 'new normal.' Peace must be regarded as the norm toward which the human race continually strives."

But what to do with detainees not charged with crimes - such as some of those held at the Guantanamo Bay U.S. Naval Base in Cuba - might still be a slow process, Johnson said.

In one of 25 footnotes to the written remarks, Johnson cited a U.S. Supreme Court ruling from 1948 that allowed the detention of German nationals for six years after fighting with Germany ended.

Johnson is the first Obama administration official to say clearly that ideological kinship alone is not enough to make someone part of the armed conflict, said Karen Greenberg, director of the Center on National Security at Fordham University in New York.

"You cannot just have the ideas associated with al Qaeda and be considered part of al Qaeda. It has to be an organizational tie."

The speech shows the administration could move in a new direction after Obama was reelected on November 6, she said.

The timing "has everything to do with the election and maybe we'll see some real headway in terms of ending this war, ending this emergency state," she said.

Johnson was both optimistic and legally correct in saying the conflict would eventually end, said William Banks, a Syracuse University law professor

"It has not been said in this decade-plus since 9/11, that we might be looking at that posture of a state of peace as the default," he said.